1. Field of the Invention
The present invention generally relates to shading correction, and more particularly to shading correction used in a scanner device or an image forming apparatus.
2. Discussion of the Background
The quality of a scanned image is affected by a number of factors. In one example of such a factor, a lens is incorporated in a scanner device to project light reflected by an original document onto an image sensor, such as a charged coupled device (CCD). However, due to vignetting of the lens, light transmitted through the center portion of the lens is of higher intensity than light transmitted through the edges of the lens. As a result, the corners of the scanned image appear to be darker than the central portion of the image. In another example of such a factor, non-uniform illumination of a light source, which is incorporated in a scanner device, may cause variations in brightness of the scanned image.
In order to improve the quality of the scanned image, a scanner device is usually provided with a shading correction unit, capable of applying shading correction to light reflected from an original document.
FIG. 1 illustrates a lens block 100, which is incorporated in a background scanner device. The lens block 100 is provided with a mask 102, which is defined by the circumference line of a bone-shaped opening made at a central portion of the lens block 100. The lens block 100 is fixed on a lens block stay (not shown) via a fixer 101, which is a side edge portion of the lens block 100.
The mask 102 includes two shading portions 102a facing each other, and two connecting portions 102b facing each other. The shading portions 102a partly cover a central light flux 103. The connecting portions 102b connect the shading portions 102a, without covering any portion of side light fluxes 104 and 105. With this configuration, as shown in FIG. 1, the mask 102 can function as a shading correction unit.
However, the shape of the mask 102 creates another problem. As shown in FIG. 1, the distance D1, which is a minimum distance between the connecting portion 102b and the fixer 101, is reduced. This causes the lens block 100 to easily oscillate. This further causes the problem of pixel shifting or color shifting, especially when the size of the lens block 100 is relatively small.